Known automatic transmissions for automotive vehicles include step ratio controls for effecting speed ratio changes in response to changing driving conditions. The term “speed ratio”, for purposes of this description, is defined as transmission input shaft speed divided by transmission output shaft speed.
An upshift occurs when the driving conditions require a ratio change from a lower numbered ratio (high speed ratio) to a higher number ratio (low speed ratio) in the transmission gearing. Similarly, a downshift occurs when the driving conditions require a ratio change from a higher numbered ratio (low speed ratio) to a lower number ratio (high speed ratio). The gearing can include, for example, either a planetary type gear system or a lay shaft type gear system. An automatic gear ratio shift is achieved by friction torque establishing devices, such as multiple disk clutches and multiple disk brakes. The friction torque establishing devices include friction elements, such as multiple plate clutches and band brakes, which can be actuated hydraulically.
A step-ratio automatic transmission uses multiple friction elements for automatic gear ratio shifting. A ratio change occurs in a synchronous clutch-to-clutch shift as one friction element, which may be referred to as the oncoming clutch (OCC), is engaged and a second friction element, which may be referred to as the off-going clutch (OGC), is disengaged. Failure to properly coordinate the engagement of the OCC with the disengagement of the OGC can be perceived by the vehicle occupants as an unpleasant shift event. More particularly, early engagement of the OCC relative to the release of the OGC can result in a phenomenon called tie-up. On the other hand, if the OCC is engaged too late relative to the release of the OGC, an engine flare can occur.